Foil bearing

ABSTRACT

A foil bearing includes an outer member ( 11 ), and a plurality of foils ( 13 ) that are mounted to an inner circumferential surface ( 11   a ) of the outer member ( 11 ) and directly face the inner circumferential surface ( 11   a ) of the outer member ( 11 ) in a radial direction without interposition of another member (such as back foils). The foils ( 13 ) each include: holding portions ( 13   a,    13   b ) that are formed at both circumferential ends and held while in contact with the outer member ( 11 ); and a body portion ( 13   c ) that is formed circumferentially between the holding portions ( 13   a,    13   b ) and has a bearing surface (A). At least an end portion on one side in a circumferential direction of the body portion ( 13   c ) is raised radially inward with respect to the inner circumferential surface ( 11   a ) of the outer member ( 11 ).

TECHNICAL FIELD

The present invention relates to a foil bearing.

BACKGROUND ART

Main shafts of a gas turbine and a turbocharger are driven to rotate athigh speed. Further, turbine blades mounted to the main shafts areexposed to high temperature. Thus, bearings for supporting those mainshafts are required to endure severe environments involving hightemperature and high speed rotation. As bearings for such use, anoil-lubricated rolling bearing or a hydraulic dynamic pressure bearingmay be used. However, use of those bearings is restricted under suchconditions that lubrication with a liquid such as a lubricating oil isdifficult, that an auxiliary device of a lubricating oil circulatorysystem is difficult to arrange separately in view of energy efficiency,and that shearing resistance of the liquid causes problems. Under thecircumstance, attention has been focused on an air dynamic pressurebearing as a bearing suited to use under the above-mentioned conditions.

In general, the air dynamic pressure bearing has rigid bearing surfaceson both of a rotary side and a fixed side. However, in the air dynamicpressure bearing of this type, when stability limit is exceeded under astate in which management of radial bearing gaps that are formed betweenthe bearing surfaces on the rotary side and the fixed side isinsufficient, self-excited centrifugal whirling of a main shaft, whichis called a whirl, is liable to occur. Thus, it is important to managethe gaps in accordance with operating rotation speeds. In particular, inenvironments involving drastic temperature changes as in the case of thegas turbine and the turbocharger, widths of the radial bearing gapsfluctuate due to thermal expansion, and hence the gaps are significantlydifficult to manage with high accuracy.

There has been known a foil bearing as a bearing that is less liable tocause the whirl and allows the gaps to be easily managed even in theenvironments involving drastic temperature changes. The foil bearing hasbearing surfaces formed of flexible thin films (foils) having lowflexural rigidity and supports a load by allowing the bearing surfacesto be deflected. Normally, an inner circumferential surface of thebearing is formed of a thin plate called a top foil, and a spring-likemember called a back foil is arranged on a radially outer side thereof.With this, a load on the top foil is elastically supported by the backfoil. In this case, during rotation of the shaft, an air film is formedbetween an outer circumferential surface of the shaft and an innercircumferential surface of the top foil. With this, the shaft issupported in a non-contact manner.

The foils of the foil bearing are flexible, and hence appropriate radialbearing gaps are formed in accordance with operating conditions such asa rotation speed of a shaft, a load on the shaft, and an ambienttemperature. Therefore, the foil bearing has a feature of excellentstability, and hence can be used at higher speed in comparison withgeneral air dynamic pressure bearings. Further, radial bearing gaps inthe general dynamic pressure bearings need to be managed on an order ofone thousandth of the diameter of the shaft. For example, in a shafthaving a diameter of approximately several millimeters, the radialbearing gaps of approximately several micrometers need to be constantlysecured. Thus, in consideration of not only a manufacturing tolerancebut also the thermal expansion in the drastic temperature changes, thegaps are difficult to strictly manage. Meanwhile, the foil bearing isadvantageous in that radial bearing gaps only need to be managed to havea size of approximately several tens of micrometers, and hence the foilbearing can be easily manufactured and the bearing gaps can be easilymanaged.

As examples of such foil bearings, there have been publicly known a foilbearing in which the back foil includes cut-and-raised parts so as toelastically support the top foil (Patent Literature 1), a foil bearingin which a bearing foil is elastically supported by an elastic bodyformed of wires that are woven into a mesh form (Patent Literature 2), afoil bearing in which the back foil includes support portions that areheld in contact with an inner surface of an outer race and are immovablein a circumferential direction, and elastic portions that areelastically deflected by contact pressure from the top foil (PatentLiterature 3), and the like. Further, in each of Patent Literatures 4and 5, there is disclosed what is called a multi-arc foil bearing inwhich a plurality of foils are arrayed in the circumferential direction,and both circumferential ends of each of the foils are mounted to theouter member.

CITATION LIST

Patent Literature 1: Japanese Patent Application Laid-open No.2002-364643

Patent Literature 2: Japanese Patent Application Laid-open No.2003-262222

Patent Literature 3: Japanese Patent Application Laid-open No.2009-299748

Patent Literature 4: Japanese Patent Application Laid-open No.2009-216239

Patent Literature 5: Japanese Patent Application Laid-open No.2006-57828

SUMMARY OF INVENTION Technical Problems

As described above, the related-art foil bearings each include themember for imparting elasticity to the top foil (such as the back foiland the elastic body). Thus, cost of manufacturing this member, and alarge number of man-hours for assembly of this member to the outermember are required.

In view of the circumstances, it is a first object of the presentinvention to reduce the number of components of a foil bearing so as toreduce a manufacturing cost and assemblyman-hours.

Further, in the multi-arc foil bearings disclosed in Patent Literatures4 and 5, radially inward projecting portions (displacement suppressingportions and engaging mechanisms) are formed at a plurality of positionsspaced apart from each other in the circumferential direction on theinner circumferential surface of the outer member, and the foils arefixed circumferentially between those projecting portions. However, inthis case, radially inner surfaces of the projecting portions areexposed circumferentially between the foils. Thus, areas of the bearingsurfaces are reduced by an amount corresponding to areas of theprojecting portions, and hence supportability may be reduced.

In view of the circumstances, it is a second object of the presentinvention to provide a multi-arc foil bearing comprising an outer memberthat allows foils to be mounted thereto without reducing areas ofbearing surfaces.

Still further, in the foil bearings of Patent Literatures 4 and 5, boththe circumferential ends of each of the foils are struck against theradially inward projecting portions on the inner circumferential surfaceof the outer member (displacement suppressing portions 62 in PatentLiterature 4 and ridges 70 in Patent Literature 5). With this, both thecircumferential ends of each of the foils are held by a foil holder.

It is a third object of the present invention to cause a multi-arc foilbearing to exert a greater effect of damping vibration of a shaft.

Solution to Problems

In order to achieve the above-mentioned first object, according to thefirst invention of the present application, there is provided a foilbearing, comprising: an outer member that has an inner circumferentialsurface having a cylindrical surface shape; and a plurality of foilsthat are mounted to the inner circumferential surface of the outermember and directly face the inner circumferential surface of the outermember in a radial direction without interposition of another member,the foil bearing being configured to radially support a shaft insertedon an inner circumference of the outer member in a freely relativelyrotatable manner, wherein the plurality of foils each comprise: holdingportions that are formed at both circumferential ends and held while incontact with the outer member; and a body portion that is formedcircumferentially between the holding portions and has a bearingsurface, and wherein at least an end portion on one side in acircumferential direction of the body portion of each of the pluralityof foils is raised radially inward with respect to the innercircumferential surface of the outer member.

In this way, according to the foil bearing of the first invention of thepresent application, at least the end portion on the one side in thecircumferential direction of the body portion of the each of theplurality of foils is not formed along the inner circumferential surfaceof the outer member, but is raised radially inward with respect to theinner circumferential surface of the outer member. With this, a radialgap is formed between the end portion of the body portion of the each ofthe plurality of foils and the inner circumferential surface of theouter member. Thus, when the shaft is relatively rotated to increasepressures in bearing gaps between an outer circumferential surface ofthe shaft and radially inner surfaces (bearing surfaces) of theplurality of foils, the plurality of foils are each elastically deformedin a direction in which the radial gap is narrowed. An elastic force isgenerated at this time, and thus resilience is imparted to the pluralityof foils. Thus, without back foils or elastic bodies to be interposedbetween the plurality of foils and the outer member, required resiliencecan be imparted to the plurality of foils, and hence the back foils andthe like can be omitted. As a result, the number of components of thefoil bearing can be reduced.

In the foil bearing described above, for example, at least the holdingportion on the one side in the circumferential direction of the each ofthe plurality of foils is inserted to a fixing groove formed in theinner circumferential surface of the outer member. With this, theholding portion can be held by the outer member. When this fixing grooveis inclined radially outward to the one side in the circumferentialdirection, the end portion of the body portion of the each of theplurality of foils is raised from the inner circumferential surface ofthe outer member at an inclination angle of the fixing groove. Thus, arising angle of the body portion of the each of the plurality of foilscan be adjusted through adjustment of the inclination angle of thefixing groove.

Further, when the holding portion on the one side in the circumferentialdirection of the each of the plurality of foils comprises a projectingportion formed by extending an axial partial region of the body portionto the one side in the circumferential direction, and when theprojecting portion is inserted to the fixing groove, an elastic force tobe applied to the plurality of foils can be adjusted through adjustmentof an axial width of the projecting portion. Further, when the holdingportion comprises a plurality of the projecting portions formed apartfrom each other in an axial direction, the elastic force to be appliedto the plurality of foils can be adjusted through adjustment of axialwidths or the number of the plurality of projecting portions.

When at least one of the holding portions of the each of the pluralityof foils is slidable against the outer member, frictional energy isgenerated by sliding between the plurality of foils and the outermember. With this, vibration to be caused by relative rotation of theshaft can be damped. In this case, through adjustment of frictionalcoefficients of sliding surfaces of the plurality of foils and the outermember, bearing characteristics can be adjusted. Specifically, when thefrictional coefficients of the sliding surfaces are set to be high, thefrictional energy to be generated by the sliding against the outermember is increased. Thus, a greater effect of damping the vibration tobe caused during the relative rotation of the shaft can be obtained. Asa result, stability of the bearing is enhanced. Meanwhile, when thefrictional coefficients of the sliding surfaces are set to be low, undera state in which the shaft is held in contact with the plurality offoils, contact pressure therebetween facilitates sliding of theplurality of foils against the outer member. Thus, the plurality offoils are easily deformed in conformity with the outer circumferentialsurface of the shaft. In this way, contact areas between the shaft andthe plurality of foils are increased. As a result, abrasion resistancesthereof are enhanced.

In the foil bearing described above, through adjustment of the risingangle at the end portion of the body portion of the each of theplurality of foils, areas of the bearing surfaces that generate positivepressure in a fluid in radial bearing gaps R can be adjusted. Forexample, as illustrated in the developed view of FIG. 6, when a risingangle θ1 at an end portion on the one side in the circumferentialdirection of a body portion 13 c of each foil 13 with respect to aninner circumferential surface 11 a of an outer member 11 is set to belarger than a rising angle θ2 at an end portion on another side in thecircumferential direction, a position at which a radial gap between aradially outer surface 13 c 1 of each of the foils 13 and the innercircumferential surface 11 a of the outer member 11 is largest(hereinafter referred to as peak position P) is set on the one side inthe circumferential direction (left side in FIG. 6) with respect to acircumferential central portion O of the body portion 13 c of the eachof the foils 13. In this state, when the shaft 6 is relatively rotatedto the one side in the circumferential direction (direction of thearrow), a wedge-like radial bearing gap R to be gradually narrowedtoward a forward side in a rotational direction is formed between aregion on each of the foils 13 on the another side in thecircumferential direction with respect to the peak position P and theouter circumferential surface 6 a of the shaft 6, and this regionfunctions as a bearing surface A that generates the positive pressure.In this way, when θ1>θ2 is established so that the peak position P isset rather on the one side in the circumferential direction, the bearingsurface A is formed to be large. As a result, supportability can beenhanced. Note that, in this case, the rotational direction of the shaft6 is limited to the one side in the circumferential direction (directionof the arrow in FIG. 6).

Meanwhile, as illustrated in the developed view of FIG. 8, when therising angles θ1 and θ2 at both the ends of the body portion 13 c of theeach of the foils 13 are set to be equal to each other, the peakposition P is set to the circumferential central portion O of the eachof the foils 13. With this, the bearing surface A and a bearing surfaceA′ can be formed to be equal to each other in area on both the sides inthe circumferential direction with respect to the peak position P. Thus,the equivalent supportability can be exerted irrespective of the sidesto which the shaft is rotated.

Further, in order to achieve the above-mentioned second object,according to the second invention of the present application, there isprovided a foil bearing, comprising: an outer member that has an innercircumferential surface having a cylindrical surface shape; and aplurality of foils that are mounted to the inner circumferential surfaceof the outer member, the foil bearing being configured to radiallysupport a shaft inserted on an inner circumference of the outer memberin a freely relatively rotatable manner, wherein the plurality of foilseach have a bearing surface, and comprise both circumferential endportions that are held while in contact with the outer member, whereincircumferential end portions of adjacent two of the plurality of foilsare intersected with each other in an axial view, and wherein thecircumferential end portions of the each of the plurality of foils arearranged on a radially outer side with respect to adjacent foils.

In this way, when the circumferential end portions of the adjacent twoof the plurality of foils are intersected with each other in the axialview, and when the circumferential end portions of the each of theplurality of foils are arranged on the radially outer side with respectto the adjacent foils, the circumferential end portions of the each ofthe plurality of foils can be held by the inner circumferential surfaceof the outer member on a back side (radially outer side) of theplurality of foils. With this, the plurality of foils can be arrangedcontinuously with each other in the circumferential direction, and theinner circumferential surface of the outer member can be covered overallwith the plurality of foils. As a result, a decrease in area of thebearing surface can be avoided.

For example, when both the circumferential end portions of the each ofthe plurality of foils are inserted to fixing grooves formed in theinner circumferential surface of the outer member, the plurality offoils can be mounted to the outer member. Alternatively, also when theend portion on the one side in the circumferential direction of the eachof the plurality of foils is inserted to the fixing groove formed in theinner circumferential surface of the outer member while the end portionon the another side in the circumferential direction of the each of theplurality of foils is arranged between the adjacent foil and the innercircumferential surface of the outer member, the plurality of foils canbe mounted to the outer member.

When at least the end portion on the one side in the circumferentialdirection of the each of the plurality of foils is slidable against theouter member, frictional energy is generated by sliding between theplurality of foils and the outer member. With this, vibration to because by relative rotation of the shaft can be damped.

Specifically, when the end portion on the one side in thecircumferential direction of the each of the plurality of foilscomprises a projecting portion formed by extending an axial partialregion, and when the end portion on the another side in thecircumferential direction of the each of the plurality of foilscomprises another projecting portion formed by extending another axialregion different from that for the projecting portion, the projectingportion and the another projecting portion formed at the end portions ofthe adjacent foils can be intersected with each other in the axial view.Alternatively, also when the end portion on the one side in thecircumferential direction of the each of the plurality of foilscomprises the projecting portion formed by extending the axial partialregion, and when the end portion on the another side in thecircumferential direction of the each of the plurality of foilscomprises a slit so that the projecting portion is inserted to the slitof the adjacent foil, the end portions of the adjacent foils can beintersected with each other in the axial view. In those cases, when theprojecting portions are formed on the end portion on the one side in thecircumferential direction of the each of the plurality of foils at aplurality of positions spaced apart from each other in an axialdirection, and when those projecting portions are mounted to the outermember, the plurality of foils can be held with a good balance in theaxial direction.

The foil bearing described above may comprise elastic members (such asback foils) for imparting radially inward elasticity to the plurality offoils, the elastic members being interposed between the plurality offoils and the inner circumferential surface of the outer member.

Further, in order to achieve the above-mentioned third object, accordingto the third invention of the present application, there is provided afoil bearing, comprising: an outer member that has an innercircumferential surface having a cylindrical surface shape; and aplurality of foils that are mounted to the inner circumferential surfaceof the outer member and directly face the inner circumferential surfaceof the outer member in a radial direction without interposition ofanother member, the foil bearing being configured to radially support ashaft inserted on an inner circumference of the outer member in a freelyrelatively rotatable manner, wherein the plurality of foils eachcomprise: holding portions that are formed at both circumferential endsand held while in contact with the outer member; and a body portion thatis formed circumferentially between the holding portions and has abearing surface, and wherein the holding portions are each held in aslidable state in a fixing groove formed in the inner circumferentialsurface of the outer member.

Advantageous Effects of Invention

As described above, according to the foil bearing of the first inventionof the present application, the back foils or the elastic bodies forimparting the elasticity to the foils (top foils) having the bearingsurfaces can be omitted. Thus, the number of components can be reduced,with the result that a manufacturing cost and assembly man-hours can bereduced.

Further, according to the second invention of the present application,it is possible to provide a multi-arc foil bearing comprising the outermember that allows the foils to be mounted thereto without reducing theareas of the bearing surfaces.

Still further, according to the foil bearing of the third invention ofthe present application, the holding portions of the foils and thefixing grooves are slid against each other. With this sliding of thefoils, a greater effect of damping the vibration of the shaft can beexerted.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view of a configuration of a gas turbine.

FIG. 2 is a sectional view of a support structure for a rotor of the gasturbine.

FIG. 3 is an axial front view of a foil bearing according to anembodiment of the present invention.

FIG. 4 (a) is a perspective view of a foil that is used in the foilbearing.

FIG. 4 (b) is a perspective view of a state in which a plurality of thefoils of FIG. 4(a) are combined with each other.

FIG. 5 is an enlarged sectional view of the foil bearing.

FIG. 6 is a developed view of the foil bearing in which acircumferential direction is converted into a straight direction.

FIG. 7 (a) is a view of a state in which a shaft and the foil are heldin contact with each other under a state in which frictionalcoefficients of sliding surfaces of the foil and an outer member are setto be high.

FIG. 7 (b) is a view of a state in which the shaft and the foil are heldin contact with each other under a state in which the frictionalcoefficients of the sliding surfaces of the foil and the outer memberare set to be low.

FIG. 8 is an enlarged sectional view of a foil bearing according toanother embodiment of the present invention.

FIG. 9 is a developed view of the foil bearing of FIG. 8 in which thecircumferential direction is converted into the straight direction.

FIG. 10(a) is a perspective view of a foil that is used in a foilbearing according to still another embodiment of the present invention.

FIG. 10 (b) is a perspective view of a state in which a plurality of thefoils of FIG. 10(a) are combined with each other.

FIG. 11 is an enlarged sectional view of the foil bearing according toyet another embodiment of the present invention.

FIG. 12 is a developed view of the foil bearing of FIG. 11 in which thecircumferential direction is converted into the straight direction.

FIG. 13(a) is a perspective view of a foil that is used in a foilbearing according to yet another embodiment of the present invention.

FIG. 13 (b) is a perspective view of a state in which a plurality of thefoils of FIG. 13(a) are combined with each other.

FIG. 14 is an axial front view of a foil bearing according to yetanother embodiment of the present invention.

FIG. 15 (a) is a perspective view of a back foil that is used in thefoil bearing of FIG. 14.

FIG. 15 (b) is a perspective view of a state in which a plurality of theback foils of FIG. 15(a) are combined with each other.

FIG. 16 is an enlarged sectional view of the foil bearing of FIG. 14.

FIG. 17 is a schematic side view of a configuration of a supercharger.

DESCRIPTION OF EMBODIMENTS

Now, description is made of embodiments of the present invention withreference to the drawings.

FIG. 1 is a schematic view of a configuration of a gas turbine apparatuscalled a gas turbine. The gas turbine mainly comprises a turbine 1comprising a blade cascade, a compressor 2, a power generator 3, acombustor 4, and a regenerator 5. The turbine 1, the compressor 2, andthe power generator 3 comprise a common shaft 6 extending in ahorizontal direction. The shaft 6, the turbine 1, and the compressor 2serve as an integrally rotatable rotor. Air sucked from an air-intakeport 7 is compressed by the compressor 2, heated by the regenerator 5,and then fed into the combustor 4. The compressed air is mixed with fueland combusted so as to rotate the turbine 1 with a high-temperature andhigh-pressure gas. A rotational force of the turbine 1 is transmitted tothe power generator 3 through intermediation of the shaft 6 so as torotate the power generator 3. In this way, electric power is generatedand output through intermediation of an inverter 8. The gas havingrotated the turbine 1 has a relatively high temperature. Thus, the gasis fed into the regenerator 5 so that heat thereof is exchanged withthat of the compressed air prior to the combustion. In this way, theheat of the gas after the combustion is reused. The gas that has beensubjected to the heat exchange in the regenerator 5 passes through anexhaust heat recovery device 9, and then is exhausted as an exhaust gas.

FIG. 2 is an illustration of an example of a support structure for therotor of the gas turbine. This support structure comprises radialbearings 10 arranged at two positions in an axial direction, and thrustbearings 20 and 20 arranged on both sides in the axial direction of aflange portion 6 b of the shaft 6. The radial bearings 10 and the thrustbearings 20 support the shaft 6 in a freely rotatable manner in a radialdirection and both thrust directions.

In this support structure, a region between the turbine 1 and thecompressor 2 is adjacent to the turbine 1 that is rotated by thehigh-temperature and high-pressure gas, and hence a temperature of anatmosphere therein is high. In this high-temperature atmosphere, alubricant such as a lubricating oil and grease is deteriorated andevaporated. Thus, normal bearings (such as a rolling bearing) that usesuch lubricants are difficult to apply. Thus, air dynamic pressurebearings, in particular, foil bearings are suited to the bearings 10 and20 that are used in the support structure of this type.

Now, description is made of the configuration of the foil bearing 10that is suited to the radial bearings for the gas turbine with referenceto the drawings.

As illustrated in FIG. 3, this foil bearing 10 comprises an outer member11 that is fixed to an inner circumference of a housing (not shown) andhas an inner circumference on which the shaft 6 is inserted, and aplurality of foils 13 mounted to an inner circumferential surface 11 aof the outer member 11. The foil bearing 10 is a foil bearing of what iscalled a multi-arc type in which the inner circumferential surface 11 aof the outer member 11 is formed into a cylindrical surface shape, andthree foils 13 are arrayed in a circumferential direction on the innercircumferential surface 11 a. Between the inner circumferential surface11 a of the outer member 11 and the foils 13, members for impartingelasticity to the foils 13 (such as back foils) are not interposed, anda radially outer surface 13 c 1 of each of the foils 13 and the innercircumferential surface 11 a of the outer member 11 directly face eachother in a radial direction.

The foils 13 each comprise holding portions 13 a and 13 b formed at bothcircumferential ends thereof, and a body portion 13 c formedcircumferentially between both the holding portions 13 a and 13 b. Thefoils 13 are each formed to integrally comprise the holding portions 13a and 13 b and the body portion 13 c through, for example, press workingof a single foil. The holding portions 13 a and 13 b are held while incontact with the outer member 11. The holding portions 13 a and 13 b ofadjacent foils 13 are formed to intersect with each other in an axialview (refer to FIG. 3). The holding portions 13 a and 13 b of each ofthe foils 13 are arranged on a radially outer side with respect to thebody portions 13 c of the adjacent foils 13. In the illustrated example,the holding portions 13 a and 13 b are inserted to fixing grooves 11 band 11 c formed in the inner circumferential surface 11 a of the outermember 11. The fixing grooves 11 b and 11 c are formed, for example, bya wire cutting process over an entire axial length of the outer member11. At least one of the holding portions 13 a and 13 b is not perfectlyfixed to corresponding one of the fixing grooves 11 b and 11 c, and heldin a slidable state. The fixing groove 11 b is inclined radially outwardto one side in the circumferential direction (forward side in arotational direction of the shaft 6, refer to the arrow in FIG. 3), andthe fixing groove 11 c is inclined radially outward to another side inthe circumferential direction. The fixing grooves 11 b and 11 c areopened at the same position in the circumferential direction. The bodyportion 13 c of each of the foils 13 is formed by curving a rectangularflat plate into a substantially circular-arc shape, and has a radiallyinner surface 13 c 2 as a bearing surface A.

As illustrated in FIG. 4 (a), the holding portion 13 a on the one sidein the circumferential direction of each of the foils 13 comprises aprojecting portion formed by extending an axial partial region of thebody portion 13 c (axial central portion in the illustrated example) tothe one side in the circumferential direction. Meanwhile, the holdingportions 13 b on the another side in the circumferential direction ofeach of the foils 13 each comprise a projecting portion formed byextending another axial part of the body portion 13 c to the anotherside in the circumferential direction. The holding portions 13 b on theanother side in the circumferential direction comprise a plurality of(two in the illustrated example) projecting portions formed apart fromeach other in the axial direction, and a recessed portion 13 b 1 isformed axially therebetween. The holding portion 13 a formed at one endof the foil 13 is inserted to the recessed portion 13 b 1 between theholding portions 13 b formed at another end of the adjacent foil. Withthis, the holding portions 13 a and 13 b intersect with each other inthe axial view (refer to FIG. 4 (b)).

As illustrated in FIG. 4 (b), the holding portion 13 a on the one sideof each of the foils 13 is inserted to the recessed portion 13 b 1formed axially between the holding portions 13 b on the another side ofthe adjacent foil 13 so that the plurality of (three in the illustratedexample) foils 13 are integrated with each other. In this state, theholding portions 13 a and 13 b are inserted respectively to the fixinggrooves 11 b and 11 c. With this, the plurality of foils 13 are mountedto the inner circumferential surface 11 a of the outer member 11. Inthis way, when the holding portions 13 a and 13 b of adjacent foils 13are intersected with each other and inserted to the fixing grooves 11 band 11 c on the radially outer side (back side) of the foils 13, anentire circumference of the inner circumferential surface 11 a of theouter member 11 can be covered with the body portions 13 c of the foils13. Thus, areas of the bearing surfaces can be maximally secured.Further, circumferential end portions of each of the foils 13 (holdingportions 13 a and 13 b) are not exposed to sliding surfaces of the foils13 and the shaft 6. Thus, a risk that the circumferential end portionsof each of the foils 13 are curled up radially inward can be reliablyprevented.

As illustrated in FIG. 5, both circumferential ends of the body portions13 c of the foils 13 (boundary portion between the holding portions 13 aand 13 b) do not extend along the inner circumferential surface 11 a ofthe outer member 11, but are raised radially inward towardcircumferential centers of the body portions 13 c with respect to theinner circumferential surface 11 a. In the illustrated example, boundaryportions between both the circumferential ends of the body portion 13 cand the holding portions 13 a and 13 b are smoothly continued withoutbeing bent, and the circumferential end portions of the body portion 13c are each curved to form a radially inward convex. With this, at boththe circumferential ends of the body portions 13 c of the foils 13,radial gaps δ are formed between the radially outer surfaces 13 c 1 ofthe foils 13 and the inner circumferential surface 11 a of the outermember 11. The foils 13 are elastically deformable in a direction inwhich the radial gaps δ are narrowed. Specifically, the foils 13 aredesigned to be deformed only elastically (not to be plasticallydeformed) at the time when the radial gaps δ are narrowed by the foils13 that are pressed radially outward by an increase in pressure in aradial bearing gap R between an outer circumferential surface 6 a of theshaft 6 and the bearing surface A of each of the foils 13 along withrotation of the shaft 6. When vicinities of end portions of the bodyportions 13 c of the foils 13 are elastically deformed in this way, aradially inward elastic force is applied to the foils 13. The elasticforce to be applied to the foils 13 can be adjusted through adjustmentof axial dimensions, the number, and arrangement positions of theholding portions 13 a and 13 b, or as described below, rising angles θ1and θ2 at both the ends of the body portions 13 c.

In this embodiment, the rising angles at both the ends of the bodyportion 13 c of each of the foils 13 with respect to the innercircumferential surface 11 a of the outer member 11 are different fromeach other. The rising angle at the end portion of the body portion 13 crefers to an angle between a tangent at the end portion of the bodyportion 13 c and a tangent at a position on the inner circumferentialsurface 11 a of the outer member 11, at which the inner circumferentialsurface 11 a of the outer member 11 intersects with the foil 13 (thatis, opening portion of the fixing groove 11 b). In this embodiment, asillustrated in FIG. 5, the rising angle θ1 at one end of the bodyportion 13 c (holding portion 13 a side) is set to be higher than therising angle θ2 at another end thereof (holding portions 13 b side). Inthis way, when the rising angles at both the ends of the body portions13 c satisfy θ1>θ2, as illustrated in the developed view of FIG. 6, apeak position (radially innermost swelling position) P of each of thefoils 13 is set on the one end side (holding portion 13 a side) withrespect to a circumferential central portion O of the body portion 13 cof the foil 13. When the shaft 6 is rotated to the one side in thecircumferential direction (refer to the arrow in FIG. 6), the radialbearing gap R to be gradually narrowed toward the forward side in therotational direction is formed between the outer circumferential surface6 a of the shaft 6 and the bearing surface A of the foil 13. At thistime, as described above, the peak positions P of the foils 13 are setrather on the one side, and hence the bearing surfaces A that generatepositive pressure are each formed over a large region on the another endside with respect to the peak position P. Thus, the radial bearing gap Rcan be secured in a large region in the circumferential direction. As aresult, supportability in the radial direction is enhanced. Note that,in this embodiment, the connecting portions between the body portion 13c and the holding portions 13 a and 13 b of the foil 13 are smoothlycontinued. Thus, the rising angles θ1 and θ2 of the body portion 13 care substantially equal to inclination angles of the fixing grooves 11 band 11 c.

When the rising angles θ1 and θ2 of the body portion 13 c of each of thefoils 13 are excessively high, the foils 13 largely swell radiallyinward. As a result, there arises a high risk that the foils 13 are bentby interference with the shaft 6. Thus, it is desired that the risingangles θ1 and θ2 each be set to 30° or less, preferably, to 20° or less.

The foil 13 is formed of a belt-like foil made of a metal that isexcellent in resiliency and processability, such as a steel material ora copper alloy, and having a thickness of from approximately 20 μm to200 μm. As in this embodiment, in the air dynamic pressure bearing thatuses air as a fluid film, a lubricating oil does not exist in anatmosphere, and hence a rust inhibiting effect by an oil cannot beexpected. Carbon steel and brass can be taken as typical examples of thesteel material and the copper alloy. However, general carbon steel isliable to corrode due to rust, and brass may be subjected to delayedcracking due to processing strain (this liability becomes higher inproportion to a Zn content in brass). Thus, it is preferred that thebelt-like foil be made of stainless steel or bronze.

In the configuration described above, when the shaft 6 is rotated to theone side in the circumferential direction (direction of the arrow inFIG. 3), that is, in a shrinkage direction of the wedge-like radialbearing gap R, air films are formed between the bearing surfaces A ofthe foils 13 and the outer circumferential surface 6 a of the shaft 6.When pressures of those air films become higher, the end portions of thebody portions 13 c of the foils 13 are pressed radially outward andelastically deformed in the direction in which the radial gaps δ arenarrowed. When the body portions 13 c are pressed radially outward inthis way, a moment force is applied to boundary portions between thebody portions 13 c and the holding portions 13 a and 13 b. At this time,the holding portions 13 a and 13 b are inserted to the fixing grooves 11b and 11 c, and hence both surfaces of each of the holding portions 13 aand 13 b are held by inner walls of the fixing grooves 11 b and 11 c.With this, angles of the holding portions 13 a and 13 b are maintained,and hence the rising angles at both the circumferential end portions ofthe body portion 13 c continuous therewith are also maintained. In thisstate, when the end portions of the body portion 13 c are curved,elastic forces can be effectively exerted.

Then, wedge-like radial bearing gaps R are formed at a plurality ofpositions (three positions in the illustrated example) in thecircumferential direction around the shaft 6, and the shaft 6 issupported in a freely rotatable manner in the radial direction under anon-contact state with respect to the foils 13. In this state, shapes ofthe foils 13 are maintained at positions at which the elastic forces ofthe foils 13 and the pressures of the air films to be formed in theradial bearing gaps R are counterbalanced. Note that, widths of theradial bearing gaps R, which are actually as fine as approximatelyseveral tens of micrometers, are illustrated on an exaggerated scale inFIG. 3 and FIG. 6. Further, the foils 13 are flexible, and hence thebearing surfaces A of the foils 13 are arbitrarily deformed inaccordance with operating conditions such as a load, a rotation speed ofthe shaft 6, and an ambient temperature. Thus, the radial bearing gaps Rare automatically adjusted to have appropriate widths in accordance withthe operating conditions. As a result, even under severe conditionsinvolving high temperature and high speed rotation, the radial bearinggaps R can be managed to have optimum widths, and hence the shaft 6 canbe stably supported.

In the foil bearing 10, air films formed during low speed rotationimmediately before stop of the shaft 6 or immediately after actuation ofthe shaft 6 are difficult to have a thickness equal to or larger thansurface roughnesses around the bearing surfaces A of the foils 13 andthe outer circumferential surface 6 a of the shaft 6. Thus, metalcontact occurs between the bearing surfaces A of the foils 13 and theouter circumferential surface 6 a of the shaft 6, which causes anincrease in torque. In order to reduce a frictional force generated atthis time so that the torque is reduced, it is desired that coating(first coating) for reducing surface friction be formed on any one orboth of the bearing surface A (radially inner surface 13 c 2) of each ofthe foils 13 and a surface of a member that slides against the bearingsurface A (in this embodiment, the outer circumferential surface 6 a ofthe shaft 6). As the coating, there may be used, for example, a DLCfilm, a titanium aluminum nitride film, or a molybdenum disulfide film.The DLC film and the titanium aluminum nitride film can be formedthrough CVD or PVD, and the molybdenum disulfide film can be easilyformed through spraying. In particular, the DLC film and the titaniumaluminum nitride film are hard. Thus, when the coating is formed ofthose films, abrasion resistance of the bearing surfaces A can also beenhanced. As a result, a bearing life can be prolonged.

Further, during operation of the bearing, due to influence of the airfilms formed in the radial bearing gaps, the foil 13 is radiallyexpanded overall to press the inner circumferential surface 11 a of theouter member 11. In accordance therewith, slight circumferential slidingoccurs between the radially outer surface 13 c 1 of each of the foils 13and the inner circumferential surface 11 a of the outer member 11 andbetween the holding portions 13 a and 13 b of the foils 13 and thefixing grooves 11 b and 11 c. As a countermeasure, coating (secondcoating) is formed on any one or both of the radially outer surface 13 c1 of each of the foils 13 and the inner circumferential surface 11 a ofthe outer member 11, which is held in contact with the radially outersurface 13 c 1, or on any one or both of the holding portions 13 a and13 b of the foils 13 and the fixing grooves 11 b and 11 c, which areheld in contact with the holding portions 13 a and 13 b. With this,abrasion resistances at those sliding portions can be enhanced. Notethat, in the foil bearing 10 described above, both the circumferentialends of the body portion 13 c of each of the foils 13 are raisedradially inward with respect to the inner circumferential surface 11 aof the outer member 11. Thus, the radially outer surface 13 c 1 of thebody portion 13 c and the inner circumferential surface 11 a of theouter member 11 are scarcely held in contact with each other. Thus, itis effective to form the above-mentioned second coating on the holdingportions 13 a and 13 b of each of the foils 13 and on the fixing grooves11 b and 11 c to be held in contact therewith.

Further, through adjustment of frictional coefficients of slidingsurfaces of the foils 13 and the outer member 11, bearingcharacteristics can be adjusted. Specifically, when a frictionalcoefficient of at least one of the sliding surface of each of the foils13 or the sliding surface of the outer member 11 (for example, frontsurfaces of the holding portions 13 a and 13 b of each of the foils 13)is set to be high, frictional energy to be generated by sliding againstthe outer member 11 (for example, inner surfaces of the fixing grooves11 b and 11 c) is increased. Thus, a greater effect of damping vibrationto be caused by the rotation of the shaft 6 can be obtained. In thiscase, it is preferred that the DLC film and the titanium aluminumnitride film, which are larger in friction coefficient but higher inabrasion resistance than the molybdenum disulfide film, be used as thesecond coating. Specifically, the molybdenum disulfide film is used asthe first coating to be formed on the bearing surfaces A, and thetitanium aluminum nitride, the DLC film, or the like is used as thesecond coating to be formed on the sliding portion between the foil 13and the outer member 11. With this, frictional coefficients of both thecoatings can be set to be different from each other. As a result, lowertorque and higher vibration damping property can be simultaneouslyobtained.

Meanwhile, when the frictional coefficient of at least one of thesliding surface of each of the foils 13 or the sliding surface of theouter member 11 (for example, front surfaces of the holding portions 13a and 13 b of each of the foils 13) is set to be low, abrasionresistance of the foils 13 at the time of contact sliding against theshaft 6 can be enhanced. Specifically, when the shaft 6 and the foils 13come into contact with each other during low speed rotation immediatelyafter actuation of the shaft 6 or immediately before stop of therotation of the shaft 6, contact pressure therebetween causes theholding portions 13 a and 13 b of the foils 13 to slide against thefixing grooves 11 b and 11 c of the outer member 11, and to cause thefoils 13 to be deformed in conformity with the outer circumferentialsurface 6 a of the shaft 6. At this time, when the frictionalcoefficients of the sliding surfaces of each of the foils 13 and theouter member 11 are set to be high, as illustrated in FIG. 7(a), theholding portions 13 a and 13 b of the foil 13 are difficult to slideagainst the fixing grooves 11 b and 11 c. Thus, the foil 13 is difficultto conform to the outer circumferential surface 6 a of the shaft 6. Inthis case, a contact region T1 between the foil 13 and the outercircumferential surface 6 a of the shaft 6 is small, and hence contactpressure per unit area becomes higher. As a result, the foil 13 and theshaft are liable to be abraded. Meanwhile, when the frictionalcoefficients of the sliding surfaces of each of the foils 13 and theouter member 11 are set to be low, as illustrated in FIG. 7(b), theholding portions 13 a and 13 b of the foil 13 are easy to slide againstthe fixing grooves 11 b and 11 c. Thus, the foil 13 is easy to conformto the outer circumferential surface 6 a of the shaft 6. In this case, acontact region T2 between the foil 13 and the outer circumferentialsurface 6 a of the shaft 6 is large, and hence contact pressure per unitarea becomes lower. As a result, the foil 13 and the shaft 6 are lessliable to be abraded.

The present invention is not limited to the embodiment described above.In the case of the embodiment described above, the rising angles θ1 andθ2 at both the ends of the body portion 13 c of each of the foils 13 areset to be different from each other. However, as illustrated, forexample, in FIG. 8, the rising angles θ1 and θ2 may be set to be equalto each other (θ1=θ2). In this case, as illustrated in FIG. 9, the peakposition P is set to the circumferential central portion O of each ofthe foils 13. In this state, the shape of each of the foils 13 issymmetrical in the circumferential direction with respect to the peakposition P, and the bearing surface A and a bearing surface A′ areformed to be equal to each other in area on both circumferential sideswith respect to the peak position P. With this, when the shaft 6 isrotated to the one side (refer to the solid-line arrow in FIG. 9), theradial bearing gap R to be gradually narrowed toward the forward side inthe rotational direction is formed between the bearing surface A formedon the another side in the circumferential direction with respect to thepeak position P and the outer circumferential surface 6 a of the shaft6. Meanwhile, when the shaft 6 is rotated to the another side (refer tothe dotted-line arrow in FIG. 9), a radial bearing gap R′ to begradually narrowed toward the forward side in the rotational directionis formed between the bearing surface A′ formed on the one side in thecircumferential direction with respect to the peak position P and theouter circumferential surface 6 a of the shaft 6. In this way, when therising angles θ1 and θ2 are set to be equal to each other, the samesupportability can be exerted irrespective of the sides to which theshaft 6 is rotated.

The embodiment illustrated in FIG. 10 is different from the embodimentdescribed above in that a slit 13 d is formed in the boundary betweenthe body portion 13 c and the holding portions 13 b on the another endside of the foil 13. The slit 13 d is formed at the same axial positionas that of the holding portion 13 a on the one end side of the foil 13,and is capable of allowing the holding portion 13 a to be insertedthereto (refer to FIG. 10 (b)). The holding portion 13 a is inserted tothe slit 13 d of the adjacent foil 13 so that the plurality of foils 13are coupled to each other to form a ring. In this state, the holdingportions 13 a and 13 b of each of the foils 13 are inserted to thefixing grooves 11 b and 11 c of the outer member 11. With this, thefoils 13 are mounted to the inner circumferential surface 11 a of theouter member 11. An axial view of the foil bearing 10 is the same asthat in FIG. 3. In this case, the holding portion 13 a of each of thefoils 13 is inserted to the fixing groove 11 b through the slit 13 d ofthe adjacent foil 13, and hence the holding portion 13 a and the slit 13d are engaged with each other in the circumferential direction. Withthis, movement in the circumferential direction of the another end(holding portion 13 b) of each of the foils 13 can be reliablyrestricted.

The embodiment illustrated in FIG. 11 is different from the embodimentsdescribed above in that the fixing groove 11 c is not formed in theouter member 11, and the holding portion 13 b on the another end side ofthe foil 13 is formed along the inner circumferential surface 11 a ofthe outer member 11. Specifically, the holding portions 13 a and 13 b ofadjacent foils 13 are intersected with each other in the axial view(refer to FIG. 4 (b) or 10 (b)). In this state, the holding portion 13 aon the one end side is inserted to the fixing groove 11 b of the outermember 11, and the holding portion 13 b on the another end side of eachof the foils 13 is inserted to a space between the adjacent foil 13 andthe inner circumferential surface 11 a of the outer member 11. Withthis, the holding portion 13 b on the another end side of each of thefoils 13 is pressed from a radially inner side by the adjacent foil 13,and held while in contact with the inner circumferential surface 11 a ofthe outer member 11. Note that, also in this case, as in the embodimentdescribed above, through adjustment of the frictional coefficients ofthe sliding surfaces of the holding portions 13 b of the foils 13 andthe inner circumferential surface 11 a of the outer member 11, bearingcharacteristics can be adjusted (refer to FIG. 7).

In this case, movement of the foils 13 to the one side in thecircumferential direction (direction of the solid-line arrow in FIG. 11)is restricted when the holding portion 13 a strikes against a depthportion of the fixing groove 11 b or when the slit strikes against theholding portion 13 a. Meanwhile, the foils 13 are movable to the anotherside in the circumferential direction (direction of the dotted-linearrow in FIG. 11). Thus, when the shaft 6 is rotated to the another sidein the circumferential direction (direction of the dotted-line arrow inFIG. 11), the foils 13 slide against the shaft 6 and are moved to theanother side in the circumferential direction. As a result, the holdingportion 13 a on the one end side of each of the foils 13 may bedisengaged from the fixing groove 11 b. Further, in this foil bearing10, the holding portion 13 b on the another end side of each of thefoils 13 is formed along the inner circumferential surface 11 a of theouter member 11. Thus, as illustrated in FIG. 12, the rising angle θ2 onthe another end side of the body portion 13 c is substantially zero. Asa result, the peak position P of the foil 13 is set rather on the oneend side with respect to the circumferential central portion O of thebody portion 13 c (left side in FIG. 12). Thus, when the shaft 6 isrotated to the one side in the circumferential direction (direction ofthe solid-line arrow), on the radially outer surface 13 c 1 of the bodyportion 13 c, the large region on the another end side with respect tothe peak position P (right side in FIG. 12) functions as the bearingsurface A. In view of this, this foil bearing 10 is used for supportingthe shaft 6 that is relatively rotated only to the one side in thecircumferential direction (direction of the solid-line arrow).

The embodiment illustrated in FIG. 13 is different from the embodimentsdescribed above in that the holding portion 13 a on the one side in thecircumferential direction of the foil 13 comprises a plurality ofprojecting portions spaced apart from each other in the axial direction.When the plurality of projecting portions (holding portions 13 a) spacedapart from each other in the axial direction in this way are inserted tothe fixing groove 11 b of the outer member 11, the end portions of thefoil 13 are held at two positions spaced apart from each other in theaxial direction in the fixing groove 11 b. Thus, the foil 13 can be heldwith a good balance in the axial direction. In this embodiment, as inthe embodiment illustrated in FIG. 10, the slits 13 d are formed at theanother end of the foil 13 so that the holding portions 13 a areinserted to those slits 13 d. However, the present invention is notlimited thereto, and although not shown, as in the embodimentillustrated in FIG. 4, the holding portions 13 b at the another end ofthe foil 13 may comprise a plurality of projecting portions extendedfrom the body portion 13 c so that the holding portions 13 a on the oneside are inserted to a recessed portion between those holding portions13 b. Further, the foils 13 may be fixed to the outer member 11 byinserting, as illustrated in FIG. 3, both the holding portions 13 a and13 b to the fixing grooves 11 b and 11 c, or by inserting, asillustrated in FIG. 11, only the holding portions 13 a on the one sideto the fixing groove 11 b while forming the holding portions 13 b on theanother side along the inner circumferential surface 11 a of the outermember 11. Still further, in FIG. 13, two projecting portions are formedas the holding portions 13 a. However, the present invention is notlimited thereto, and three or more projecting portions may be formed asthe holding portions 13 a. In this case, the slits 13 d are formed asmany as the projecting portions as the holding portions 13 a.

The foil bearing 10 illustrated in FIG. 14 comprises the outer member 11that is fixed on the inner circumference of the housing (not shown) andhas the inner circumference on which the shaft 6 is inserted, aplurality of foils 13 (top foils) mounted to the inner circumferentialsurface 11 a of the outer member 11, and elastic members interposedbetween the inner circumferential surface 11 a of the outer member 11and the foils 13 so as to impart elasticity to the foils 13. In the casedescribed in this embodiment, back foils 12 correspond to the elasticmembers. The circumferential end portions of the adjacent foils 13 (inthis embodiment, holding portions 13 a and 13 b) are formed to intersectwith each other in the axial view (refer to FIG. 16). The holdingportions 13 a and 13 b of each of the foils 13 are arranged on theradially outer side with respect to the body portion 13 c of theadjacent foil 13.

As illustrated in FIG. 15 (a), the back foil 12 has substantially thesame shape as that of the foil 13, and comprises holding portions 12 aand 12 b formed at both circumferential ends thereof, and a body portion12 c formed circumferentially between both the holding portions 12 a and12 b. The holding portions 12 a and 12 b are each formed into asubstantially flat plate shape, and the body portion 12 c is formed intoa shape that can be elastically deformed by a compressive force in theradial direction. In this embodiment, the body portion 12 c is formedinto a corrugated shape, and a height between ridges and furrows thereinis gradually reduced from a circumferential central portion toward bothends of the body portion 12 c. Similarly to the foils 13, in each of theback foils 12, the holding portion 12 a formed at one end can beinserted between a pair of holding portions 12 b formed at another endof the adjacent back foil 12 (refer to FIG. 15 (b)).

The foil bearing 10 is assembled as follows. First, three foils 13 eachhaving substantially the same shape (refer to FIG. 4) and three backfoils 12 each having substantially the same shape (refer to FIG. 15) areprepared and superimposed on each other in three pairs. Then, theholding portions 13 a and 12 a at the one end of the foil 13 and theback foil 12 superimposed on each other are inserted between pairs ofthe holding portions 13 b and 12 b formed at the another end of anotherpair of the foil 13 and back foil 12 so that the three pairs of thefoils 13 and the back foils 12 are coupled to each other to form a ring.In this state, the holding portions 13 a and 12 a on the one side andthe holding portions 13 b and 12 b on the another side in each of thepairs of the foils 13 and the back foils 12 are inserted to the fixinggrooves 11 b and 11 c of the outer member 11. With this, the foils 13and the back foils 12 are mounted to the inner circumferential surfaceof the outer member 11.

In the configuration described above, when the shaft 6 is rotated to theone side in the circumferential direction (direction of the arrow inFIG. 14), that is, in a shrinkage direction of the wedge-like radialbearing gap R, air films are formed between the bearing surfaces A ofthe foils 13 and the outer circumferential surface 6 a of the shaft 6.When the pressures of the air films become higher, the end portions ofthe body portion 13 c of each of the foils 13 are pressed to theradially outer side. As a result, the back foils 12 are elasticallydeformed and compressed in the radial direction. Then, the wedge-likeradial bearing gaps R are formed at a plurality of positions (threepositions in the illustrated example) in the circumferential directionaround the shaft 6, and the shaft 6 is supported in a freely rotatablemanner in the radial direction under a non-contact state with respect tothe foils 13. In this state, the shapes of the foils 13 and shapes ofthe back foils 12 are maintained at positions at which elastic forces ofthe back foils 12 and the pressures of the air films to be formed in theradial bearing gaps R are counterbalanced. Note that, the widths of theradial bearing gaps R, which are actually as fine as approximatelyseveral tens of micrometers, are illustrated on an exaggerated scale inFIG. 14. Further, the foils 13 and the back foils 12 are flexible, andhence the bearing surfaces A of the foils 13 are arbitrarily deformed inaccordance with the operating conditions such as the load, the rotationspeed of the shaft 6, and the ambient temperature. Thus, the radialbearing gaps R are automatically adjusted to have appropriate widths inaccordance with the operating conditions. As a result, even under thesevere conditions involving high temperature and high speed rotation,the radial bearing gaps R can be managed to have optimum widths, andhence the shaft 6 can be stably supported.

In the foil bearing 10, the air films formed during the low speedrotation immediately before the stop of the shaft 6 or immediately afterthe actuation of the shaft 6 are difficult to have the thickness equalto or larger than the surface roughnesses around the bearing surfaces Aof the foils 13 and the outer circumferential surface 6 a of the shaft6. Thus, the metal contact occurs between the bearing surfaces A of thefoils 13 and the outer circumferential surface 6 a of the shaft 6, whichcauses an increase in torque. In order to reduce the frictional forcegenerated at this time so that the torque is reduced, it is desired thatthe coating (first coating) for reducing surface friction be formed onthe any one or both of the bearing surface A (radially inner surface 13c 2) of each of the foils 13 and the surface of the member that slidesagainst the bearing surface A (in this embodiment, the outercircumferential surface 6 a of the shaft 6). As this coating, there maybe used, for example, the DLC film, the titanium aluminum nitride film,or the molybdenum disulfide film. The DLC film and the titanium aluminumnitride film can be formed through CVD or PVD, and the molybdenumdisulfide film can be easily formed through spraying. In particular, theDLC film and the titanium aluminum nitride film are hard. Thus, when thecoating is formed of those films, the abrasion resistance of the bearingsurfaces A can also be enhanced. As a result, a bearing life can beprolonged.

Further, during operation of the bearing, due to the influence of theair films formed in the radial bearing gaps, the foils 13 and the backfoils 12 are radially expanded overall to press the innercircumferential surface 11 a of the outer member 11. In accordancetherewith, the slight circumferential sliding occurs between theradially outer surfaces 13 c 1 of the foils 13 and radially innersurfaces of the back foils 12, between radially outer surfaces of theback foils 12 and the inner circumferential surface 11 a of the outermember 11, and between the holding portions 13 a, 13 b, 12 a, and 12 bof the foils 13 and the back foils 12 and the fixing grooves 11 b and 11c. As a countermeasure, the coating (second coating) is formed on anyone or both of the radially outer surfaces 13 c 1 of the foils 13 andthe radially inner surfaces of the back foils 12, which are held incontact with the radially outer surfaces 13 c 1, on any one or both ofthe radially outer surfaces of the back foils 12 and the innercircumferential surface 11 a of the outer member 11, which is held incontact with the radially outer surfaces, on any one or both of theholding portions 13 a and 13 b of the foils 13 and the fixing grooves 11b and 11 c, which are held in contact with the holding portions 13 a and13 b, or on any one or both of the holding portions 12 a and 12 b of theback foils 12 and the fixing grooves 11 b and 11 c, which are held incontact with the holding portions 12 a and 12 b. With this, abrasionresistances at those sliding portions can be enhanced.

Note that, in order to exert a greater vibration damping effect, asomewhat great frictional force may be necessary at the above-mentionedsliding portions. Thus, frictional property of the second coating neednot be significantly low. For those reasons, it is preferred that theDLC film and the titanium aluminum nitride film, which are larger infrictional coefficient but higher in abrasion resistance than themolybdenum disulfide film, be used as the second coating. Specifically,the molybdenum disulfide film is used as the first coating to be formedon the bearing surfaces A, and the titanium aluminum nitride, the DLCfilm, or the like is used as the second coating to be formed on thesliding portion between the foil 13 and the outer member 11. With this,frictional coefficients of both the coatings can be set to be differentfrom each other. As a result, lower torque and higher vibration dampingproperty can be simultaneously obtained.

In the foil bearing 10 of FIG. 14, the foils 13 illustrated in FIGS. 10,11, and 13 may be used. In this case, the back foils 12 are the same asthe foils 13 of FIG. 10 except that the body portions 12 c arecorrugated (not shown).

It is preferred that the back foils 12 have substantially the same shapeas that of the foils 13 as described above. Thus, for example, when thefoils 13 have the shapes illustrated in FIGS. 4 and 10, it is preferredthat the back foils 12 have substantially the same shape as thoseshapes. Note that, in a case where the back foils 12 are mounted to theouter member 11 by means other than that for the foils 13, the backfoils 12 need not have substantially the same shape as that of the foils13.

Further, the elastic members are not limited to the back foils 12 aslong as radially inward elasticity can be imparted to the foils 13. Forexample, there may be used an elastic body formed of wires that arewoven into a mesh form.

In the cases described hereinabove, the three foils 13 are provided inthe foil bearing 10, but the present invention is not limited thereto.Two foils 13, or four or more foils 13 may be provided.

Further, in the cases exemplified in the description hereinabove, theshaft 6 serves as a rotary side member, and the outer member 11 servesas a fixed side member. However, the configuration of FIG. 3 may beapplicable as it is also to a reverse case where the shaft 6 serves asthe fixed side member, and the outer member 11 serves as the rotary sidemember. Note that, in this case, the foils 13 serve as rotary sidemembers, and hence the foils 13 need to be designed in consideration ofcentrifugal deformation of the foils 13 as a whole.

The foil bearing 10 according to the present invention is applicable notonly to the gas turbine described above, and may be used as a bearingfor supporting a rotor of a supercharger, for example. As illustrated inFIG. 17, in the supercharger, a turbine 51 is driven with an exhaust gasgenerated in an engine 53, and a compressor 52 is rotated by a driveforce thus generated, to thereby compress intake air. This configurationenables the engine 53 to generate higher torque and have higherefficiency. The turbine 51, the compressor 52, and the shaft 6 serve asa rotor, and the foil bearings 10 of the embodiments described above canbe used as the radial bearings 10 for supporting the shaft 6.

The foil bearing according to the present invention can be used not onlyturbo-machines such as the gas turbine and the supercharger, but widelyused also as bearings for vehicles such as an automobile, which are usedunder restrictions such as a difficulty in lubricating with a liquidsuch as a lubricating oil, a difficulty in separately arranging anauxiliary device of a lubricating oil circulatory system in view ofenergy efficiency, or problems that may be caused by shearing resistanceof the liquid. The foil bearing according to the present invention canbe widely used also as bearings for industrial devices.

Further, the foil bearings described above are each an air dynamicpressure bearing that uses air as a pressure generating fluid. However,the present invention is not limited thereto, and other gases or liquidssuch as water and oil may be used as the pressure generating fluid. Inaddition, unlike the exemplified cases where any one of the shaft 6 andthe outer member 11 is used as a rotary side member and another of theshaft 6 and the outer member 11 is used as a fixed side member, both ofthose members may be used as rotary side members that are rotated atdifferent speeds.

REFERENCE SIGNS LIST

-   10 foil bearing-   11 outer member-   11 b, 11 c fixing groove-   13 foil-   13 a, 13 b holding portion-   13 c body portion-   A bearing surface-   O circumferential central portion of body portion of foil-   P peak position-   R radial bearing gap-   δ radial gap-   θ1, θ2 rising angle of foil

The invention claimed is:
 1. A foil bearing comprising: an outer memberthat has an inner circumferential surface having a cylindrical surfaceshape; and a plurality of foils that are mounted to the innercircumferential surface of the outer member, wherein the foil bearing isconfigured to radially support a shaft which rotates to one side in acircumferential direction with respect to the outer member, wherein theplurality of foils each comprise: holding portions that are formed atboth circumferential ends and held while in contact with the outermember; and a body portion that is formed circumferentially between theholding portions and has a bearing surface, wherein the holding portionon the one side in the circumferential direction of each of theplurality of foils is inserted to a fixing groove formed in the innercircumferential surface of the outer member, and wherein the holdingportion on another side in the circumferential direction of each of theplurality of foils is arranged between an adjacent one of the pluralityof foils and the inner circumferential surface of the outer member. 2.The foil bearing according to claim 1, wherein the fixing groove isinclined radially outward to the one side in the circumferentialdirection.
 3. The foil bearing according to claim 1, wherein the holdingportions of the each of the plurality of foils are slidable against theouter member.
 4. The foil bearing according to claim 1, wherein theholding portion on the one side in the circumferential direction of eachof the plurality of foils comprises a projecting portion formed byextending an axial partial region of the body portion to the one side inthe circumferential direction.
 5. The foil bearing according to claim 4,wherein the holding portion on the one side in the circumferentialdirection of each of the plurality of foils comprises a plurality of theprojecting portions formed apart from each other in an axial direction.6. A foil bearing comprising: an outer member that has an innercircumferential surface having a cylindrical surface shape; and aplurality of foils that are mounted to the inner circumferential surfaceof the outer member, wherein the foil bearing is configured to radiallysupport a shaft which rotates to one side in a circumferential directionwith respect to the outer member, wherein the plurality of foils eachcomprise: holding portions that are formed at both circumferential endsand held while in contact with the outer member; and a body portion thatis formed circumferentially between the holding portions and has abearing surface, wherein the holding portion on the one side in thecircumferential direction of each of the plurality of foils is insertedto a fixing groove formed in the inner circumferential surface of theouter member, and wherein the holding portion on the one side in thecircumferential direction of the each of the plurality of foilscomprises a projecting portion formed by extending an axial partialregion of the body portion to the one side in the circumferentialdirection.
 7. The foil bearing according to claim 6, wherein the holdingportion on another side in the circumferential direction of each of theplurality of foils is arranged between an adjacent one of the pluralityof foils and the inner circumferential surface of the outer member. 8.The foil bearing according to claim 6, wherein the holding portion onthe one side in the circumferential direction of the each of theplurality of foils comprises a plurality of the projecting portionsformed apart from each other in an axial direction.
 9. The foil bearingaccording to claim 6, wherein the holding portions of each of theplurality of foils are slidable against the outer member.